It is sometimes desirable to locate the position of a station operable within a wireless, e.g., radio frequency (RF), network. For example, the United States Federal Communications Commission (FCC) has decreed that cellular telephone systems must implement systems to provide mobile telephone position information for use in emergency response, e.g., enhanced 911 (E911) emergency response. Additionally, the use of global positioning system (GPS) receivers, which operate in conjunction with a network of middle earth orbit satellites orbiting the Earth to determine the receiver's position, has almost become ubiquitous in navigational applications.
In order to estimate a station's location, a system typically measures a metric that is a function of distance. The measured metric is often signal strength, which decays logarithmically with distance in free space. Time information, such as time of arrival of a signal or time difference of arrival of a signal at diverse antennas, may be utilized as a measured metric from which distance information may be determined. Typically, several reference points are used with distance information derived from the measured metric in estimating location.
In the aforementioned GPS network, for example, the reference points are the satellites and the measured metric is the time of arrival of the satellite signal to the GPS receiver. The time of arrival of the satellite signal is typically directly proportional to the distance between the satellite and the GPS receiver due to a clear line of sight between the GPS receiver and satellite. By measuring the time of arrival associated with three satellites, a GPS receiver can calculate the longitude and latitude of the GPS receiver. By using time of arrival information with respect to a fourth satellites, a GPS receiver can also determine altitude.
In the aforementioned cellular networks, techniques including signal strength measurements and/or time difference of arrival have been implemented for location determination. For example, U.S. Pat. No. 6,195,556, the disclosure of which is incorporated herein by reference, teaches the use of signal strength measurements in combination with the time difference of arrival of a station's signal in determining the location of the station. Additionally, U.S. Pat. No. 6,195,556 teaches the use of mapping of received signal characteristics associated with particular positions (e.g., receive “signature” associated with each of a plurality of remote station locations) for use in determining a station's location. In the case of the aforementioned cellular network the base transceiver stations (BTSs) are generally relied upon as the reference points from which distance determinations are made.
Wireless local area network (WLAN) location determination systems have been implemented by capturing or mapping the fingerprint (e.g., signal strength, multipath characteristics, etcetera) of wireless access points (APs) at particular points in the service area. These location fingerprints are measured off-line, such as when a network is being deployed, and are stored in a database for later comparison to the wireless signal environment as experienced by a remote station. By comparing the measured fingerprint at the current location of a remote station to the entries in the database, a nearest match may be returned as the estimated location.
The above techniques for determining station location have numerous disadvantages associated therewith. For example, the time of arrival techniques of the aforementioned GPS system rely upon highly complex and costly satellites, which include atomic clock mechanisms etcetera, to provide accurate time information. Furthermore, the proper operation of a GPS receiver requires clear line of sight to three satellites, which is often not possible in topologically complex environments, such as densely built out urban areas and indoor environments. It is well known that GPS receivers are not well suited for use indoors or even in the cabin of a boat or other vessel, unless the enclosure is constructed of suitable RF permeable material or the GPS receiver antenna is disposed external thereto. Of course, disposing the GPS receiver antenna externally may not be desirable as the location of the antenna and not the GPS receiver itself would be determined.
Topologically complex environments generally provide an environment in which the wireless channel is very noisy and wireless signals suffer from reflection, diffraction, and multipath effects. Such environments result in the received signal strength often no longer providing a direct correspondence to distance, although the attenuation of such a signal in free space is predictable with respect to propagation distance. For example, the signal strength, as measured by a network node, of signals from two stations disposed in such a topologically complex environment an equal distance from the network node, may appear to be a random value. That is, although the same transmit-receive distance to the node is present with respect to both such stations, the receive signal strength as measured by the node for each station may be substantially different due to the characteristics of wireless propagation channel.
The aforementioned mapping or fingerprinting techniques utilized with respect to WLANs may generally be relied upon to address variations in wireless propagation. However, the time and expense involved in setting up such a fingerprint database is typically significant. Moreover, implementation of such a technique involves the modification and cooperation of the remote stations for the measurement and comparison of such data.